"Ask adults from the industrialized world what number is halfway between 1 and 9, and most will say 5. But pose the same question to small children, or people living in some traditional societies, and they're likely to answer 3. Cognitive scientists theorize that that's because it's actually more natural for humans to think logarithmically than linearly." Fascinating. The human brain is such a magical machine.

My guess would be 5. My 9 year old son said 5, but then said it could also be 5.5. So five seems to be the dominant number in any answer.

More interesting (IMHO):
Yesterday I was listening to a podcast where someone said 0.999999999999-> ad inifinitum is the same as exactly 1, because there is no other number between those two numbers so they must be the same.

In practice they are basically the same thing of course, but in maths they shouldn't be, then again they are.